FIELD OF THE INVENTION AND PRIOR ART

The present invention relates to a controllable inductor comprising at least a tubular core, a main winding surround¬ ing the core and a control winding passing substantially axially through the core.

Such controllable inductors may through the main winding thereof be connected to any electrical circuit, such as an electric power line, so as to provide this circuit with an inductance, for example for extinguishing harmonic currents generated in the circuit. The magnetic permeability of the core and by that inductance of the inductor may then be con- trolled by modifying an electric control current caused to flow axially through the core in said control winding. By connecting such a controllable inductor in series with a ca¬ pacitor a so called harmonic filter is obtained, which is already known through for example WO 94/11891 of the appli- cant and in which the impedance may be controlled to be low for certain frequencies by controlling the inductance of the inductor for fading out harmonic currents having a frequency being a multiple, for example 11, of the fundamental fre¬ quency of the network.

Usually an alternating voltage is connected to the main winding, but it would also be possible to connect a direct voltage with an overlapped alternating voltage to the main winding, but in that case the inductor would only have a useful influence upon the alternating voltage part. The con¬ trol current brought to flow through the control winding is

normally a direct current, but it would just as well be pos¬ sible to use an alternating current as control current and by controlling such an alternating current control current appropriately even voltage induced in the control winding, which causes harmonic currents in the main winding and losses in the core, may be eliminated.

In controllable inductors already known the main magnetic flux extending substantially axially through the core is closed in the air outside the core and the main winding, so that a so called air reactor is formed, but a disadvantage of such an inductor is that it allows a regulation of the inductance within a comparatively narrow interval, most often only by a factor of about 10%. This narrow regulation interval of the inductance of such an inductor strongly lim¬ its its field of use, and it may therefore mainly be used as an harmonic filter.

Also other controllable inductors not having any control winding are known, but these may in principle be regarded as fixed, but through intermittently connecting different such inductors to the circuit in question it is possible to pro¬ vide it with a controllable inductance. In such fixed induc¬ tors functioning according to this principle the harmonic or overtone generation will be great with considerable disad¬ vantages resulting therefrom with respect to primarily the need of several filter banks so as to eliminate harmonics generated. Additionally, these inductors have to be con¬ trolled by thyristors to be water-cooled and by that con- trolled through an expensive control equipment.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a control- lable inductor of the type defined in the introduction, which has a simple construction and by that is inexpensive

at the same time as the function thereof is reliable and which makes it possible to regulate the inductance within a comparatively wide range for broadening the field of use of such a controllable inductor with respect to the already known inductors discussed above.

This object is according to the invention obtained by pro¬ viding such a controllable inductor with a yoke of a mate¬ rial having a high magnetic permeability arranged to extend outside the core and the main winding and together with the core form a close loop having at the most small air gaps for a main magnetic flux generated in the core by a current in said main winding and extending substantially axially in the core, and the control winding comprises first plates extend- ing substantially axially through the core and being of a material having a good electric conductivity.

Thanks to the fact that the control winding comprises said first plates it is possible to obtain a control winding at a low cost, but above everything a stable mechanical construc¬ tion of the inductor, so that it becomes possible to conduct the control winding in a path outside the main winding, by which it according to the invention is possible to arrange a yoke closing the main magnetic flux through the core with at the most small air gaps in the loop provided in this way for the main magnetic flux. The controllability gets very high in such an inductor having at the most small air gaps, since the main part of the energy stored will be within material having a low magnetic permeability, contrary to a so called air reactor in which a great part of the energy is stored in the air and by that may not be regulated just as easily. In an inductor of the type according to the invention a regula¬ tion of the inductance is therefor made possible to a con¬ siderably larger extent than in inductors of the type de- fined in the introduction already known, such as easily by a factor 5 or more. "Small air gaps" are defined as air gaps

being small with respect to the thickness of the wall of the core, so that eddy current losses may be avoided. It would for sure be possible to arrange many small such distributed air gaps in the core, when no particularly great controlla- bility of the inductance of the inductor is needed, since in this way iron may be saved and the entire inductor could be less costly. However, the greatest controllability is ob¬ tained when the air gap is a minimum.

A possible field of use for an inductor of this type is a switching in thereof in alternating voltage power lines, which have a high capacitance built-in therein, for example cable networks. By an intermediate connection of the induc¬ tor an inductance of a desired size may be added and by that the reactance of the power line may be reduced for a more efficient energy transfer through the line.

According to a preferred embodiment of the invention the control winding also comprises second plates extending out- side the core and the main winding and being of a material having a good electric conductivity, said plates being elec¬ trically connected to said first plates and adapted to to¬ gether therewith form closed loops for a current flow in the first plates through the core. By such a construction of the control winding this receives both a very stable mechanical construction, so that the function thereof will be constant over the time and reliable, and it also becomes simple and cheap to manufacture.

According to another preferred embodiment of the invention the first plates are arranged in different packages of plates pressed together with their large flat surfaces thereof against each other, said packages having one or more plates, and these plate packages have substantially the same cross section. Plates having one and the same thickness may by that be used for obtaining electric conductors, which are

constituted by the plate packages, for the control current through the core having substantially the same cross sec¬ tion, so that substantially just as much loss heat is gener¬ ated in each conductor and there will be no problem with lo- cal superheating.

According to a further development of the embodiment last mentioned the plate packages have a thickness reduced in the direction of a radius of the core perpendicularly to the large flat surfaces of the first plates towards the centre of the core for obtaining a maximum filling of the inner hollow space of the core. By such a design of the control winding, i.e. a reduction of the thickness of the control plates where these may be made wider in the direction of their large flat surfaces thereof in parallel with a radius of the core, a maximum filling of the inner hollow space of the core and by that a good controllability of the inductor may be obtained.

According to another preferred embodiment of the invention the yoke comprises plates being arranged to extend substan¬ tially in parallel with each other at each end of the core and being with their large surfaces thereof in parallel with a plane defined by a radius of the core and the axis of the core, said yoke plates have an edge thereof located in the immediate vicinity of the respective core end so as to re¬ ceive the main magnetic flux from the core without any sub¬ stantial air gap therebetween, and said second control plates are arranged in a space between yoke plates arranged side by side and have substantially the same direction in the room as the yoke plates, so the yoke plates and these control plates form a sandwich construction. This embodiment is very advantageous, since yoke plates extending substan¬ tially in parallel with each other without any real distur- bance of the second control plates included in the control winding may be brought to cover substantially the entire

area through which the main magnetic flux to be led further may be expected to cover, so that no cross flux plates are needed and a cross magnetization of the yoke and eddy cur¬ rent losses caused thereby is avoided. By the fact that said plates of the control winding are of a material having a low magnetic permeability, i.e. having a high reluctance, the reluctance perpendicular to the yoke plates may in this way be made high, so that the control flux is prevented from go¬ ing out of the core and into the yoke at the ends of the core. A control flux in the yoke would impair its permeabil¬ ity and result in increased losses.

According to a further preferred embodiment of the invention said second control plates are arranged with their large flat surfaces substantially in parallel with the large flat surfaces of the first control plate, and at least a first control plate of each control plate package is arranged to protrude from the core past the edge of the respective sec¬ ond control plate located closest to the core so as to enter into electric contact establishing bearing thereagainst. A stable closed loop of the control winding may in this way be easily formed.

According to another preferred embodiment of the invention the inductor is intended to be connected to a three-phase alternating current network and it has one core and one main winding for connection to each phase. Such an inductor is particularly advantageous since the voltages induced in the control windings through the alternating main magnetic flux will cancel each other out, so that a generation of harmon¬ ics in the network and losses in the core are avoided.

According to another preferred embodiment of the invention the inductor has, for connection to a multiphase alter- nating-current network, a yoke in common to and closing the main magnetic flux through all the cores and forming main

magnetic flux paths between all cores. This is important for keeping the main magnetic flux within the parts having a high magnetic permeability (yoke and core), since the main magnetic flux flowing through a core has to be able to be distributed on the other cores and the sum of the main mag¬ netic flux has in each moment to be zero.

According to another preferred embodiment of the invention the control winding of each core is electrically connected to a control winding for the adjacent core through said sec¬ ond control plates and the control winding of the two cores located outermost is through said control plates also elec¬ trically connected to one outer leg each of third control plates, which like the first control plates connect second control plates on one end of the cores to second control plates on the other end of the cores. By arranging such an outer leg with three control plates it may easily be accom¬ plished that a control current is running through all first control plates in all the cores, wherein an advantageous such realization is defined in the appended claim 19.

Further advantages as well as preferred features of the in¬ vention will appear from the following description and the other dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

With reference to the appended drawings, below follows a de¬ scription of preferred embodiments of the invention cited as examples.

In the drawings:

Fig 1 is a simplified, partially sectioned side view of a controllable inductor intended to be connected to a three-

phase alternating-current network according to a first pre¬ ferred embodiment of the invention,

Fig 2 is a simplified view from above of the inductor ac- cording to Fig 1,

Fig 3 is an enlarged partially schematic view from above il¬ lustrating the arrangement of yoke and control winding through a sandwich construction over one of the cores of the inductor according to Fig 1 and 2,

Fig 4 is a view from above of a part of the inductor accord¬ ing to Fig 1 and 2, said yoke having been omitted for the sake of illustration,

Fig 5 is a simplified vertical section slightly enlarged with respect to Fig 1 through a core of the inductor accord¬ ing to Fig 1,

Fig 6 is a simplified view illustrating how first and second control plates are connected to each other in the inductor according to Fig 1,

Fig 7 is a view illustrating how control plates of the in- ductor according to Fig 1 are connected to each other for obtaining the control current path illustrated,

Fig 8 is a view corresponding to Fig 1 of an inductor adapted for connection to a three-phase alternating-current network according to a second preferred embodiment of the invention,

Fig 9 is a view corresponding to Fig 2 of an inductor ac¬ cording to Fig 8, wherein, however, for illustrating purpose the control winding has been omitted,

Fig 10 is a simplified perspective view of an inductor ac¬ cording to a third preferred embodiment of the invention, which is adapted for connection to a one-phase alternating voltage, and a control winding has in this figure been omit- ted so as to better illustrate the construction of the in¬ ductor,

Fig 11 is a view corresponding to Fig 7 illustrating an al¬ ternative connection of control plates of the control wind- ing to each other for obtaining the current path illus¬ trated, and

Fig 12 illustrates schematically in a view from above how the control windings could run through one core in the con- trol-plate connection according to Fig 11.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVEN¬ TION

The inductor schematically illustrated in Fig 1, the con¬ struction of which will now be explained while at the same time referring to Fig 2, is adapted to be connected to a three-phase alternating-current network and has three main windings 1 schematically indicated, each of which is wound in layers at a distance outside a carrier not shown, such as a cylinder of electrically insulating material. Each such main winding is connected to one phase of their own of said alternating-current network and has an upper end connected to high potential, wherein the voltage is falling in the di- rection towards the opposite end being the lower one in Fig 1, which is on ground potential, but it could just as well be on potential. Inside and co-axially to the respective main winding and with an interval 2 with respect thereto a core 3 of a material having a high magnetic permeability, such as iron, is arranged. A control winding 4, which is formed by a plurality of separate part control windings and

is built in a way to be explained further below, passes sub¬ stantially axially through the respective core. This control winding runs in loops in a way to be explained further on. The control winding 4 is connected to a voltage, which most often would be a direct current voltage, but an alternating voltage is also conceivable, and this voltage gives rise to a current in the control winding. The alternating current in the main winding generates a main magnetic flux, which passes substantially axially through the core, while the control current in the control winding 4 will generate a magnetic flux directed tangentialiy and transversally to the main flow and in this way reduce the permeability thereof for the longitudinal magnetic flux from the main winding. By increasing the current in the control winding 4 the perme- ability of the core may be reduced and by that the induc¬ tance of the inductor may be reduced. This is the main prin¬ ciple according to which a controllable inductor of this type functions. This principle is already known, and the special thing with the invention is how the inductor is con- structed so as to make this controllability possible, and this will now be explained while referring to the drawing figures enclosed. It is first of all appropriate to remind of the power generation per volume unit as a consequence of a magnetic flux passing transversally to surfaces of a me- tallic object is proportional to the square of the thickness of the object measured perpendicularly to the flow direc¬ tion, which is the reason for the production of the core 3 with very thin plates wound through a plurality of turns, which however does not appear from the figure. By way of ex- ample it may here also be mentioned that the control current typically may be a direct current in the region of 100-500 A, while the high potential end of the main windings may be connected to a voltage of 400 kV. A controllable inductor of this type with so high direct currents easily results in a problem to achieve controlling of the magnetic flux in the

cores and not at another place through the direct current, and how this is solved will be explained below.

The control winding 4 comprises first plates of material having a good electric conductivity, preferably copper, which are divided into several packages 5, which each is formed by a number of thin plates 6 pressed in electric con¬ tact against each other by means of their large flat sur¬ faces, such as illustrated in Fig 4. These first plates ex- tend substantially axially through the respective core and they are arranged with their large flat surfaces substan¬ tially in parallel with each other and each package is elec¬ trically insulated from adjacent packages and the core 3 through suitable spacer pieces 7 of a material being elec- trically insulating. The plate packages 6 have in their ex¬ tension inside the core 3 substantially the same cross sec¬ tion, since they have to conduct a control current of the same magnitude and this gives rise to that the heat produc¬ tion in the respective package will be nearly the same, so that the cooling need for each package may be assured. The same current density in the plate packages results in the best utilizing of the material, so that such a small amount material as possible may be used and costs may be kept low. It is illustrated in Fig 4 how the different plate packages 5 have a thickness decreasing towards the centre of the core in the direction of a radius of the core perpendicularly to the large flat surfaces of the first plates for obtaining a maximum filling of the inner cavity of the core, and in this way a filling factor in the order of 60% of the inner cavity of the core may be obtained.

It is schematically illustrated in Fig 6 how a control plate package 6 extending through a core is connected to second plates 8 being part of the control winding, which are also of material having a good electric conductivity and prefer¬ ably is formed by similar plates with respect to the mate-

rial and possibly also the thickness of the first control plates. The second control plates 8 extend outside the core and the main winding substantially perpendicularly to the axis of the core and with their large flat surfaces thereof substantially in parallel with their large flat surfaces of the first control plates 6. At least one of the first con¬ trol plates 6' is arranged to project from the core past the edge 9 located closest to the core of the respective second control plate so as to enter into bearing thereagainst es- tablishing an electric contact. It is pointed out that it has in Fig 6 for simplifying purpose been illustrated how the control plate package 5 in the core only has four plates and two plates 6' and 6" project out from the core, but in practice each element shown as a plate in that figure would be formed by a number of plates laid against each other. Thus, the second plates 8 may also be arranged in packages 10, such as illustrated in Fig 6 by arranging two such plates against each other there. It is illustrated through Fig 6 that the thickness of the package 10 of second plates is less than the thickness of the package 5 of first control plates, which is necessary for creating a space 11 (see Fig 4) between adjacent packages 10 of second control plates for a reason to be described further below. It is then fully possible that the cross section of said package of second control plates is smaller than the cross section of the package 5 of first control plates, since a higher heat re¬ lease as a consequence of the control current may be ac¬ cepted in those portions of the control winding which are located outside the core than in those inside the core, since the former portions may be more easily cooled. In or¬ der to be able to conduct the control current further from those control plates 6 (in Fig 6 two such ones) in the pack¬ age 5 not extending through the package 10 a welding seam not shown is applied on the upper edge thereof and over to the plate 6" for conducting the current over thereto.

It is illustrated in Fig 4 how the packages 5 of the first control plates are arranged in two groups 12, 13 which are separated from each other by a space 14 extending transver¬ sally to the large flat surfaces thereof. This space is ar- ranged so as to offer a possibility of good cooling of the control winding by passing a cooling medium, such as oil, air or the like, therethrough (see also the middle arrow in Fig 5) . The respective package 10 of second control plates is at the respective end thereof only connected to first control plates belonging to one of the two groups and ex¬ tends there only to said space 14. The control winding of each core is in this way arranged through the packages 10 of second plates 8 electrically connected to a control winding for the adjacent core, i.e. the packages of first plates 6 running through that core, and the control winding of the two cores located outermost are through said package 10 also electrically connected to an outer leg 15, 16 (see Fig 1) each of third control plates 17, which extend in a corre¬ sponding way as the first control plates and connect second control plates at one end of the cores to second control plates at the other end of the cores. The third control plates are also arranged in packages consisting of one or several thin plates, which are electrically insulated from each other, the number of such packages being just as high as the number of packages of second control plates.

The different packages of first, second and third control plates are so connected to each other that a current path (see Fig 7) is formed from a first 15 of the outer legs, at which the control winding is connected to a control voltage, to the first control plates of the core located closest thereto and back to the outer leg so many times that all the first control plates of one control plate group 12 of this core has been passed, then further to the adjacent second core for running through a loop through the first control plates of this core and the first control plates of the ad-

jacent third core so many times that all the first control plates of one control plate group of the second and third cores have been passed, and so on until the second outer leg 16 is reached and then back to the first outer leg while running through all the first control plates of the second control plate group 13 of the respective core. The perme¬ ability of all the three cores and by that the inductance of the inductor may by that be controlled by simple means through one single connection to a control voltage source.

Yoke plates 18 or packages of such yoke plates are arranged in the space 11 between adjacent control plate packages 10, said yoke plates being of a material having a high magnetic permeability, preferably iron, and extend from one outer core to the other outer core. The yoke plates and the con¬ trol plate packages 10 form then a sandwich construction, such as illustrated in Fig 3. Accordingly, the spaces 11 are provided so as to enable arrangement of such yoke plates therein, these yoke plates being omitted in Fig 4. Said yoke plates 18 are also arranged in the direction perpendicularly to the large flat surfaces of the plates outside the second control plates of the packages, such as illustrated in Fig 2, and the yoke plates 19 located there are arranged without any space therebetween. The yoke plates are by that arranged to cover at least substantially the entire cross section of the core and the space 2 between the core and the main wind¬ ing. This definition is intended to comprise the case illus¬ trated that the yoke plates are arranged with a certain space therebetween, where the packages 10 of second control plates are located. It is then in the practice possible to have another relation between the thickness of the yoke plates 18 and the packages 10 of second control plates dif¬ ferent from what is illustrated in Fig 3. An insulating layer 20 is arranged between the yoke plates 18 and adjacent control plate packages 10.

It is illustrated in Fig 5 how the second control plates 8 are arranged to have the edge 9 thereof located closest to the respective core end at a distance from this core for al¬ lowing a passage of a cooling medium, such as air, oil or the like, from the interior of the core and radially out¬ wardly therefrom at said core end, as is indicated through the arrows, while the yoke plates 18 extend by the edge 21 thereof located closest to the core 3 at a very small dis¬ tance from the core 3 for obtaining a minimum air gap 22 therebetween.

The different cores are magnetically connected to each other through the yoke plates 18, 19, and the longitudinal main magnetic flux formed in the respective core 3 may be closed through these yoke plates and the other cores included in the inductor in a substantially air gap free way, so that the main part of the energy in the inductor will be stored within this "iron", so that the inductance of the inductor may be controlled within a wide range, which easily may mean a controllability by a factor 5. The main magnetic flux lines coming out of the respective core 3 directly under a package of second plates 10 have to be slightly bent for en¬ tering the yoke plate located closest thereto, which results in a certain concentration of flux lines there, which how- ever is a small problem. Thanks to the fact that the pack¬ ages 10 of second control plates, which have a high reluc¬ tance, are arranged between yoke plates where such packages 10 are present, the cross control magnetic flux running in the core is efficiently prevented from going up into the yoke plates extending more or less transversally thereto and then downwardly into the core again, so that it is effi¬ ciently avoided that the control flux magnetizes the yoke plates and by that deteriorates the permeability thereof.

The yoke plates 18, 19 cover, except for the entire respec¬ tive core, also a space 2 between the main winding and the core for absorbing leakage flux present there.

The main advantages of an inductor according to Fig 1 are the following:

1. No cross flux plates are required for absorbing the mag¬ netic flux from the entire respective core and the space lo- cated between the core and the main winding, which results in an inexpensive construction.

2. No cross magnetization of the yoke through the control current takes place, which would procure hysteresis losses and eddy current losses in the yoke.

3. The control winding may be produced to a low cost, and it is here underlined that, although it is spoken about wind¬ ing, it is a question of comparatively stiff bodies as far as the control plate packages are concerned.

4. The construction will be very stable.

5. The voltages in the control winding induced by the main voltage in the main winding cancel each other out in an in¬ ductor of this type connected to a three-phase alternating- current network.

An inductor according to another embodiment of the invention is illustrated in Fig 8 and 9, the construction of which corresponds to a large extent to that of the inductor ac¬ cording to Fig 1-7, so that here only the main differences therebetween will be explained. Corresponding parts of this inductor have been provided with the same reference numerals as for the inductor according to Fig 1-7. This inductor dif¬ fers from that according to Fig 1 by second plates 8 not

shown in Fig 9 run in the region directly above the inner hollow space 23 of the cores, while there are no longitudi¬ nal yoke plates, but such 19 are only located on both sides of the second control plates 8. This means in its turn that the longitudinal yoke plates will not cover the entire re¬ spective core end and the space between the main winding and the respective core for receiving the main magnetic flux coming from the respective core end, as a consequence of which transversal yoke plates 24 are arranged closer to the core than the longitudinal yoke plates 19 and arranged to cover at least substantially the entire core at the respec¬ tive end thereof for conducting the main magnetic flux from the core up to the longitudinal yoke plates 19. This induc¬ tor functions in essentially the same way as that according to the first embodiment, but a disadvantage of this with re¬ spect to the first one is that the cross flux yoke plates 24 may cause a part of the control magnet flux to be led up thereinto, so that both longitudinal and cross magnetization of the yoke through the control current may occur. The yoke may by that be saturated with increased iron losses as a consequence.

Furthermore, an inductor according to a third embodiment of the invention is illustrated in Fig 10, which is intended to be connected to a one-phase voltage, and this has four sub¬ stantially U-shaped yoke pieces 25 arranged with a division of 90° and arranged to close the main magnetic flux at the respective core end. These leave an opening for the inner hollow space 23 of the core therebetween for passing a con- trol winding not shown therebetween. By arranging the yoke pieces 25 with air gaps 26 therebetween, the risk for an in¬ fluence of the control current upon the permeability of the "iron" is reduced by the fact that control flux from the core goes up into the yoke pieces, which is advantageous.

It is illustrated in Fig 11 that also other control winding alternatives than that illustrated primarily in Fig 7 are conceivable. Thus, the current runs all the turns through each core 28, 29, 30 on the way from one single outer leg 27 to the most remote third core 30 and the current is then running directly back to the connection to the voltage source in question at the outer leg 27. How the first core then may be realised is shown simplified in Fig 12. No transversal space between control winding halves consisting of control plate packages is present here, but only longitu¬ dinal spaces 31 for receiving yoke plates between the con¬ trol plate packages 5. The longitudinal control plate pack¬ ages 10 are only shown through lines, but they have a simi¬ lar construction as those illustrated in i.a. Fig 6.

The present invention is of course not in any way restricted to the preferred embodiments described above, but many pos¬ sibilities to modifications thereof would be apparent to a man skilled in the art.

As an example of such modifications it may be mentioned that the mutual dimensioning of the different parts included in the inductor may be varied within a broad scope.

It may also be mentioned that the inductor may be manufac¬ tured for another number of phases than what has been shown in the figures.